Page 108 - Microtectonics

96 4 · Foliations, Lineations and Lattice Preferred Orientation

Box 4.8 Area and volume change leading to spaced foliation (e.g. Fig. 4.14). In most cases,
microfolds (mechanical rotation) develop in the dia-
In geological practice, it is easy to confuse area change and vol- genetic foliation and this initial stage is followed by solu-
ume change. Area change is a component of two-dimensional
strain and is measured in a plane, e.g. from stretch values; it tion transfer of material between hinges and limbs, usu-
causes a change in the cross-sectional area of a structure (e.g. a ally quartz from limbs to hinges (Williams 1972a; Cos-
fossil). Volume change is a component of three-dimensional grove 1976; Gray 1979; Waldron and Sandiford 1988), and/
strain. Area change is not a direct measure for volume change. or syntectonic crystallisation or recrystallisation of mi-
For example, even if a thin section shows evidence for area in- cas in cleavage domains (Tullis 1976; White and Knipe
crease, bulk volume loss may occur if shortening is significant
in the direction normal to the thin section. Only if strain is 1978; Knipe 1981; White and Johnston 1981; Lee et al. 1986;
two-dimensional, i.e. if stretch normal to the plane of observa- Kisch 1991). These effects are thought to be mainly tem-
tion equals 1 (plane strain), can area change be used as a meas- perature-dependent, solution transfer occurring at lower
ure of volume change (Fig. 4.32). grade than syntectonic crystallisation and recrystallisa-
tion (Kanagawa 1991; Kisch 1991). Consequently, solution
transfer may be followed by syntectonic crystallisation
served graptolites that permit the measurement of abso- (Weber 1981). With increasing temperature in the absence
lute finite strains (Goldstein et al. 1998) constitute an ex- of deformation, a preferred orientation may even be
ception. The significance of volume change may in some strengthened further by mimetic mica growth (Siddans
cases be overestimated since evidence of shortening nor- 1977; Weber 1981; Ishii 1988). In some slates, the stage of
mal to a foliation (partly dissolved structures and fossils; folding and rotation may be absent and the foliation de-
Fig. 4.21) is usually clear, while evidence of extension par- velops by syntectonic crystal growth without mechanical
allel to the foliation (e.g. fibres around pyrite, boudinaged rotation (Woodland 1982; Gregg 1985; Ishii 1988).
micas) is easily overlooked (Fig. 4.32). After a first foliation is developed, renewed shorten-
Crenulation cleavage development is probably associ- ing at a low angle to the existing foliation may cause de-
ated with volume increase of microlithons and volume velopment of a second foliation; again, the early foliation
decrease of cleavage domains while bulk deformation may may be folded or truncated by developing new cleavage
be approximately volume-constant (Fig. 4.32; Erslev and domains, and either solution transfer or new growth of
Mann 1984; Lee et al. 1986; Waldron and Sandiford 1988; mica and possibly other minerals such as plagioclase
Bhagat and Marshak 1990; Wintsch et al. 1991; Manckte- (Williams et al. 2001) may dominate. This leads to dis-
low 1994; Stewart 1997; Saha 1998). Quartz, albite and, to junctive or crenulation cleavage. If differentiation is strong
a lesser extent, micas are exchanged in pelites while zir- and accompanied by recrystallisation, evidence of early
con, apatite and rutile are largely inert (Southwick 1987; foliations may be obscured and a compositional layering
Waldron and Sandiford 1988; Williams et al. 2001). In develops. The term differentiated layering is also com-
many rocks, solution transfer may therefore only occur monly used for such structures, but since it can be diffi-
on a small scale and spacing of foliation may actually de- cult to distinguish sedimentary layering from secondary
pend on the distance over which solution transfer is ca- layering, the non-genetic term compositional layering is
pable of maintaining strain compatibility in a deforming preferred.
rock (Waldron and Sandiford 1988). In psammites, continuous foliation can form in fine-
grained rocks, or spaced foliations in coarse-grained
4.2.9.4 material (Gray 1978). In the second case, mica films
Foliations, Lithotype and Metamorphic Conditions (Fig. 4.23b) may develop by solution transfer and mica
growth (Gregg 1985) and/or by the development of mi-
Secondary foliations develop by processes mentioned in cro shear zones (Goodwin and Tikoff 2002).
Sect. 4.2.7, but in different lithotypes and under different In limestones, foliation development is strongly de-
metamorphic conditions, these processes operate to differ- pendent on temperature and mica-content. Solution
ent extents. A brief outline of present ideas is given below. transfer and twinning are important at low temperature
In pelites, mechanical rotation, pressure solution trans- (Sect. 3.12.3; Davidson et al. 1998) and can lead to a grain
fer, crystallisation, recrystallisation and oriented nucleation shape preferred orientation defined by elongated carbon-
are all competing processes. In many cases, a diagenetic fo- ate grains, or a coarse spaced foliation (stylolites). A pri-
liation may have been present before onset of foliation mary high mica content of limestone may cause develop-
development. In some cases, at very low-grade or non- ment of slaty cleavage and cleavage bundles. In one case
metamorphic conditions, cleavage domains develop ob- the growth of illite + kaolinite + quartz + anatase in cleav-
lique to the diagenetic fabric by stress-induced solution age domains was reported to accompany the removal of
transfer or development and rotation of micro shear zones calcite by dissolution (Davidson et al. 1998). Passive rota-
(Goodwin and Tikoff 2002) with no- or minimal folding, tion of micas is mainly responsible for mica-preferred